Photo-induced dichroic polarizers and fabrication methods thereof

ABSTRACT

The present invention provides a method of forming a polarizing material comprising exposing a layer of dichroic material to activating light illumination to provide an ordered structure with a distinguished absorption axis and thus photo-induce polarization, and fixing the induced polarization by polymerisation of the dichroic layer. The present invention also provides novel polarizing materials formed thereby. By selectively exposing regions of the dichroic material to differing activating radiation, different regions with different polarization axes can be created. The polarizing material can also be provided with a coating or coatings to alter the spectral response, and a stack formed of a plurality if dichroic layers may be provided.

FIELD OF THE INVENTION

[0001] This invention relates to methods of fabricating polarizers fromdichroic materials, and to polarizers fabricated by such methods. Inparticular, the invention relates to photochemically stable dichroicmolecules and the device structures, which are suitable for thefabrication of thin light polarizers.

PRIOR ART

[0002] Light-polarization films or polarizers are major componentsliquid crystal displays (LCDs) and other liquid crystal (LC) devices.Commercial polarizers are usually based on polyvinyl-alcohol-iodine(PVA) films of 150-400 μm thick. These polarizers are generally placedon the external glass surfaces of the LC cell and require protectivefilms (e.g. cellulose triacetate or cellulose acetate butyrate). Thefabrication of such known commercial polarizers is rather complicatedand expensive. Recently, to improve the cost-effectiveness, there arehave been proposals to fabricate the light polarizers by printing orphoto-alignment technologies.

[0003] In U.S. Pat. Nos. 5,739,296 arid 6,049,428, polarizing films areformed from dyestuffs, which have stable lyotropic liquid crystallinephases in a wide range of concentrations, temperatures and pH-values. Ina sheared flow, the lyotropic liquid crystal molecules areself-assembled and oriented preferentially in connection with the flowdirection. When the proper preparation conditions are met, a wellordered solid phase of the lyotropic liquid crystal is formed. Incertain cases, the order parameter as a measure of this alignment effectis high, so that this material is suitable for the fabrication ofpolarizers. To create this sheared flow, rollers and blades have beensuggested.

[0004] To minimize any defects due to the shear flow alignment, aphoto-alignment technology to prepare the thin photo-patterned polarizerhas been proposed [V. Kozenkov et al, SID'00 DIGEST, p.1099]. Since thisis a non-contact method, the particulates and static charges generatedcan be in principle eliminated. In addition, the cross contaminationproblems can be minimized. With a birefringent mask, this techniquemakes the fabrication of multi-domain structures more cost-effective forthe wide viewing-angle LCD applications

SUMMARY OF THE INVENTION

[0005] According to the present invention there is provided a method offorming a polarizing material comprising the steps of: (a) forming alayer of a dichroic material on a substrate, and (b) exposing said layerto activating light illumination to provide an ordered structure with adistinguished absorption axis. Preferably the layer may be polymerised.

[0006] Preferably different regions of the polarizing material arepolarized by activating radiation with different polarization axes so asto produce regions of said polarizing material with differing axes ofpolarization. This may be achieved by regions of the layer being exposedindependently by the use of masks to isolate selected regions forexposure. Alternatively a birefringence mask may be used to createactivating radiation with a selected spatial distribution ofpolarization vectors.

[0007] In preferred embodiments the layer may be provided with a coating(eg iodine) to change its spectral response. If different regions of thelayer are formed with different coatings a multicolor polarizingmaterial may be produced

[0008] A stack of layers may be formed on a substrate with the layersbeing separated by isolation layers.

[0009] The activating radiation is polarized or non-polarized, butdirected and may be a continuous waveform or may be pulsed. Thepolarization of the dichroic layer may be controlled by varyingparameters selected from the group consisting of the incident angle ofthe activating radiation, the exposure energy density and the processtemperature.

[0010] According to the present invention there is also provided apolarizing material comprising a layer of a photochemically stabledichroic absorber. Preferably the absorber is formed within a polymermatrix.

[0011] The dichroic absorber may be selected from the group consistingof: mono-, bis-. tris-, and poly-azo dyes, quinone dyes, mono- andpoly-oxyanthraquinone dyes, sulfur-substituted hydroxythio-anthraquinonedyes, aminohydroxy-anthraquinone dyes, anthrapyrimidinone dyes,merocyane dyes, azomethine dyes, polycyclic compounds, benzoquinones,napthoquinones, tolanes, diphenyls, p-nitroanilines,p-nitrosodialkylanilines, dialkylaminostyroles.

[0012] The polymeric materials may be selected from the group consistingof: polyimide, polyethylene, cellulose acetate, polystyrene,polycarbonate, polyester, polyacrylonitrile, polyacetal, polyacrylamide,polybutadiene, polyvinylalcohol, polymethylmethacrylate, andpolyvinylcinnamate.

[0013] The polarizing material may be provided with a coating of amaterial (eg iodine) selected to alter the spectral response of saidmaterial.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Some examples of the invention will now be described by way ofexample and with reference to the accompanying drawings, in which:

[0015]FIG. 1 shows the transmittance of an azodye film during thepumping (a) and the dark relaxation (b), the molecular transformation ispumped by a laser beam of about 100 mW/cm² at 488 nm or 514 nm, whereasthe signal is probed by a 0.2 mW-633 nm laser source,

[0016]FIG. 2 shows the absorption spectra of the exposed azodye film, A₀represents the absorbance before the illumination, and p and s are thesuffixes of p- and s-waves,

[0017]FIG. 3 shows the transmission spectra of the exposed azodye filmbefore (a) and after (b) treatment with iodine,

[0018]FIG. 4 shows an embodiment of the invention in the form of amulti-layer structure,

[0019]FIG. 5 shows an embodiment of the invention with a polarizerhaving areas of different polarization axes,

[0020]FIG. 6 shows another embodiment of a multi-axes photo-inducedpolarizer,

[0021]FIG. 7 shows embodiments of the invention with multi-colourstructures, and

[0022]FIG. 8 is an illustration of induced optical anisotropy when anazodye layer is illuminated obliquely by a polarized or non-polarizedbut directed light.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0023] Before describing a number of embodiments and examples of thepresent invention, it would be useful to define a number of terms to beused in this specification.

[0024] By “anisotropically absorbing molecules”, reference is made tocompounds with anisotropic geometry, such as rod-shaped or disc-shaped,which exhibit absorption properties with different values alongdifferent axes. For example, dichroic compounds and lyotropic liquidcrystal compounds are anisotropic absorbers.

[0025] By “linear polarized light” is meant light that is polarizedmostly along one axis (the major axis) of a plane orthogonal to thepropagation direction.

[0026] The term “photochemically active molecules” refers to moleculesthat are involved in irreversible or reversible photochemical reactions.In the solid phase, the quantum efficiency associated with thephotochemical reaction is about 0.1-1.

[0027] The term “photochemically stable molecules” refers to moleculesthat are no longer involved in the irreversible or reversiblephotochemical reaction. In the solid phase, in such cases the quantumefficiency associated with the photochemical reaction is about 10⁻⁸-0.1.

[0028] The term “photostable molecules” refers to cases where thequantum efficiency associated with the photochemical reaction is lessthan 10⁸.

[0029] By the term “photo-anisotropic medium” (PAM) is meant that theisotropic solid phases of photochemically active, photochemically stableand photostable molecules exhibit photo-induced anisotropy (absorptiondichroism and birefringence), upon the absorption of polarized ornon-polarized photons. The induced anisotropy is associated with thedirection of polarization vector, the incident angle and exposure energyof the light illumination.

[0030] The tern “substrates ” refers to any medium able to support theformation of thin surface layers of PAM for example, A substrate can beany solid combination of layered materials. The materials can be anycombination of glass, silicon, oxides, plastics and metals. Inparticular, silver, gold, aluminum, polyimide, silicon monoxide,indium-tin-oxide, silicon dioxide, and color filter layers are commonexamples.

[0031] As will be seen from the following description of preferredembodiments and examples, the present invention provides photo-inducedpolarizers formed of dichroic materials. In particular, as can be seenfrom FIG. 1(a), when the dichroic molecules are exposed to a low poweractinic radiation, long-lasting optical anisotropy and dichroism arephoto-induced.

[0032] There are two major types of physical mechanisms, which give riseto the photo-induced phenomena. The first is based on irreversiblephoto-chemical reactions, such as photo-induced cross-linking andphoto-decomposition. The anisotropic layers formed in this way arecharacterized by a small value of the order parameter (<0.4) and thecorresponding low value of the induced optical anisotropy and dichroism.The order parameter is very sensitive to the exposure time and chemicalcontent of the substance and has to be accurately controlled. Moreoverthe contamination of the initial substance by the by-products of thephoto-degradation is possible in certain cases. The dichroic spectra ofthese substances considerably change its form during the exposure. Allthese disadvantages prevent the application of the absorbing layers,formed by photo-chemical mechanism as dichroic polarizers.

[0033] The second mechanism is based on the reversible cis-transisomerization and Weigert effect, i.e. Brownian motion in a potentialfield of the actinic light. The probability ε of photo-absorption isproportional to the square of cosine angle between the polarizationvector of actinic light E_(hv) and the vectorial absorption dipoleμ_(ge), i.e. ε˜|μ_(ge)·E_(hv)|². In other words, the molecules that havetheir transition dipole moments parallel to the direction of thepolarized light will probably undergo the conformational molecularchanges. Consequently, these will lead to a non-uniform distribution ofmolecules. However, with this mechanism when the pumping light source isremoved, the molecules and domains relax slowly (FIG. 1b). The orderparameter as a measure of these photo-induced effects can be very highin some dichroic materials, but since these are reversible processes,blending with a polymer matrix or polymerization to form a network ispreferable so as to fix the alignment against any thermal or photoperturbation. Therefore, a polarizer can be produced by this clean andnon-contact method. In addition, the fabrication methods formulti-layers, -axes and -colour photo-induced polarizers based on thesame dichroic materials are described in the following. The polarizersand the polarizer fabrication techniques described in this specificationare all compatible with current LCD manufacturing techniques.

[0034] The photo-induced optical anisotropy of PAM takes place due tothe orientational molecular ordering. The major axis of inducedanisotropy is perpendicular to the direction of polarized activatingradiation. It will however be parallel to the plane of incidence for thenon-polarized actinic radiation. Some of the photochemically stablesubstances give rise to the high molecular order parameter S>0.8 and arethermally stable up to the melting temperature of the substances. Insome cases, the corresponding temperature is 140° C.-180° C. Inaddition, the spectral changes of those substances are not noticeable.

[0035] The PAMs materials can be based on photostable organic compoundsand dichroic dyes with an anisotropic absorption either in UV-, visibleor IR- spectral region in the range between 200 and 2000 nm and, inparticular, in the visible region between 400 and 800nm.

[0036] These substances belong to the following dye groups: mono-, bis-,tri- and poly-azo dyes, metal-complex azo dyes; quinone dyes; mono- andpoly-oxyanthraquinone dyes, sulphur-substitutedhydroxythio-anthraquinone dyes, aminohydroxy-anthraquinone dyes;anthrapyrimidinone dyes; merocyane dyes; azomethine dyes; polycycliccompounds; benzoquinones and naphthoquinones; tolanes; diphenyls;p-nitroanilines, p-nitrosodialkylanilines; dialkylaminostyroles etc.Besides, these substances can have bi-functional reactive groups, whichpolymerize by thermal or photo treatment to form a polymer network.Alternatively, these substances can be introduced as the chromophores inthe polymer matrix. Some examples of the polymer matrix are polyimide;polyethylene, cellulose acetate; polystyrene; polycarbonate; polyester;polyacrylonitrile; polyacetal, polyacrylamide; polybutadiene;polyvinylalcohol; polymethyl-methacrylate; polyvinylcinnamate In bothcases, the thermal, optical, electrical and mechanical properties can beimproved. Usually the concentration of the substances in the polymermatrix does not exceed 5-15 wt/wt % so that the final thickness cannotbe too thin however. The additives, which promote good adhesion,suitable viscosity and low curing temperature, can also be introduced.

[0037] Various methods can be used to put the PAM layer to thesubstrate, including spin-coating, dipping, spraying, brushing,printing, Langmuir-Blodgett technique and thermal evaporation in vacuum.

[0038] The following examples demonstrate the device structures,fabrications and applications of the photo-induced polarizers using thePAM materials. The PAM layers, obtained in the following examples, canbe deposited on rigid or flexible substrates.

EXAMPLE.1

[0039] A thin film of the following azodye (1)

[0040] is prepared by the method of vacuum sublimation at pressure ofabout 2·10⁻⁵ mm Mercury. The deposition temperature onto two glassplates is 20° C. The melting temperature of the azodye is 146° C. Thedeposition rate is 0.93 nm/sec and the total deposition time is 4.5minutes. The final thickness of the azodye layer is 0.25 microns. Duringthe deposition process, one of the glass plates is in-situ exposed to alinearly polarized light. The light is generated by a 250-Watt Mercurylamp with peak wavelength at 546 nm and the power density is 22.3mW/cm². The same polarized light exposes the second glass plate afterthe azodye layer has been formed. It is found that the photo-inducedoptical retardation of the azodye, which is measured at wavelength 632.8nm, is equal to λ/12. This is significant for many display applications.The energy densities for the first and second glass plates are about4.1J/cm² and 12.7 J/cm² respectively.

EXAMPLE. 2

[0041] Another useful azodye (2) is shown below.

[0042] To prepare the PAM layer, I-10 wt % solution of the azodye inchlorobenzene is spin-coated on a glass substrate at room temperature.This solid dye film is then irradiated at normal incidence by apolarized UV light. The light source is a 1000 W Oriel Xenon arc lamp.The UV sheet polarizer is purchased from Oriel Instruments. Theintensity of polarized V light at 365 nm is about 6 mW/cm². The energydensity is about 10.8 J/cm^(2.) In FIG. 2, the absorption spectra ofazodye (2) are shown, and the order parameter calculated in this case is0.86. FIG. 2 shows the absorption spectra prior to exposure to polarizedlight (Ao) and of the p (Ap) and s (As) waves respectively afterexposure to polarized light. The high absorption of the p wave incontrast to the s wave shows that the azodye layer is effectivelypolarized.

EXAMPLE. 3

[0043] The azodye layer can have a top coating of iodine or alcoholmolecules. Such a coating may change the spectra of the dye making itmore useful for applications in the visible light range. FIG. 3 showsthe transmission spectra of the azodye (2) before (a) and after (b) thetreatment with iodine The exposed PAM layer is prepared in accordancewith Example 2, and then the iodine molecules are evaporated at roomtemperature and atmospheric pressure. The film thickness of iodinemolecules has been measured in a control experiment. In the presentexample, a thin film of about 20nm is deposited on top of the exposedPAM lay-i The coated layer is stable against the ambient light and lowpower laboratory lasers.

EXAMPLE 4

[0044] To optimise the extinction ratio and optical transmittance, amulti-layers structure may be formed as shown in FIG. 4. The exposed PAMlayer, which has a top coating of iodine, is prepared in accordance withExample 3. However, the thickness of the PAM layer and the top coatingare reduced accordingly. To isolate this coated layer from each other, athin layer about 10 nm of polymer such as polyvinyl-alcohol is thermallyevaporated onto the iodine layer. This embodiment of the inventionimproves both the optical and mechanical properties of the resultantpolarizer.

EXAMPLE 5

[0045] The preferred orientation of the dye molecules and consequentlythe direction of the polarization axis can be independently varied indifferent surface regions. The sizes of these regions may vary fromseveral microns to tenths of centimeters. FIG. 5 shows an embodiment ofthe invention in the form of a multi-axis photo-induced polarizer, whichhas different local polarization axes in different regions. A dichroiclayer 2 is formed on a substrate 2 and is divided into a plurality ofregions 3. Each region 3 is exposed selectively by polarized light inorder to induce the polarization shown by the polarization axes 4. Thedifferent regions may be formed by masking the remainder of the azodyelayer and subjecting a selected region to a particular form of polarizedlight. Each region may thus be polarized in turn. FIG. 6 providesanother possible realization of the multi-axes polarizers. FIG. 6 showsthe result of illuminating a plate bearing a dichroic layer with axiallypolarized light and allowing the plate to rotate. This creates thepolarization distribution shown in FIG. 6. Because the dichroic layerwill be polarized in a direction perpendicular to the axis of thepolarizing light, the rotational movement of the plate results incircumferential polarization. The photographs in crossed polarizersillustrate the corresponding distribution of the intensity variations.

EXAMPLE 6

[0046] The absorption bands of the dye molecules and consequently thecolour can be independently varied in different surface regions. Thesizes of these regions vary from several microns to tenths ofcentimeters. FIG. 7 (a) and (b) show the multi-colour photo-inducedpolarizers, which have different local absorption spectra in differentregions. In the figures the letters R,G,B and D stand for red, green,blue and dark respectively as examples. This can be achieved by applyingdifferent coatings to different regions in order to provide differentspectral responses in the various regions.

EXAMPLE 7

[0047] The direction of polarization axis can be induced using theobliquely incident polarized or non-polarized light (FIG. 8). Theoptical anisotropy and dichroism depend on the exposure energy, incidentangle and process temperature. This makes possible the uniformpolarization direction on the curved surface (e.g. lens) ormicro-objects with a surface relief profile (e.g. diffraction gratings).

[0048] It will thus be seen that, at least in its preferred forms, thepresent invention provides novel device structures and fabricationtechnologies for photo-induced polarizing materials. When the dichroicmolecules are optically pumped by a polarized light beam, theprobability of their transformation is proportional to the square of thecosine θ, the angle between the transition dipole moments of themolecules and the direction of the polarized light. In other words, themolecules that have their transition dipole moments parallel to thedirection of the polarized light will probably undergo thetransformation. To minimize the dipolar absorption at the low powerdensity regime, cis-trans isomerization and/or thermal diffusion in apotential field of the actinic light occurs and both lead to anon-uniform distribution of the molecules. In certain dichroicmaterials, these give rise to long-lasting optical anisotropy anddichroism that arc able to polarize the light wave selectively. In apolymer matrix, this induced optical anisotropy and dichroism can beeven higher and kept for a long time. This can also be accomplished ifthe dichroic molecules can be polymerized by thermal or photo treatment.Therefore, a permanent polarizer can be fabricated by a non-contacttechnique that basically requires an actinic radiation source. Inpreferred forms of the invention, multi-layers, -axes and -colourpolarizers using these dichroic materials are also possible and whichmay have applications, for example, for the wide viewing-angle LCDapplications.

1. A method of forming a polarizing material comprising the steps of:(a) forming a layer of a dichroic material on a substrate, and (b)exposing said layer to activating light illumination to provide anordered structure with a distinguished absorption axis.
 2. A method asclaimed in claim 1 further comprising polymerising said layer.
 3. Amethod as claimed in claim 1 wherein different regions are polarized byactivating radiation with different polarization axes so as to produceregions of said polarizing material with differing axes of polarization.4. A method as claimed in claim 3 wherein regions of said layer areexposed independently by the use of masks to isolate selected regionsfor exposure.
 5. A method as claimed in claim 3 wherein a birefringencemask is used to create activating radiation with a selected spatialdistribution of polarization vectors.
 6. A method as claimed in claim 1wherein said layer is provided with a coating to change its spectralresponse.
 7. A method as claimed in claim 6 wherein different regions ofsaid layer are formed with different coatings to produce a multi-colorpolarizing material.
 8. A method as claimed in claim 1 comprisingforming a plurality of said layers on said substrate with said layersbeing separated by isolation layers.
 9. A method as claimed in claim 1wherein said activating radiation is polarized or non-polarized, butdirected.
 10. A method as claimed in claim 1 wherein said activatingradiation is a continuous waveform or is pulsed.
 11. A method as claimedin claim 1 wherein the polarization of the dichroic layer is controlledby varying parameters selected from the group consisting of the incidentangle of the activating radiation, the exposure energy density and theprocess temperature.
 12. A method as claimed in claim 1 wherein thedichroic layer is formed on the substrate by a method selected from thegroup consisting of spin-coating, dipping, spraying, brushing, printing,Langmuir-Blodgett technique or thermal evaporation.
 13. A polarizingmaterial comprising a layer of a photochemically stable dichroicabsorber.
 14. A material as claimed in claim 13 wherein said absorber isformed within a polymer matrix.
 15. A polarizing material as claimed inclaim 13 wherein said dichroic absorber is selected from the groupconsisting of: mono-, bis-, tris-, and poly-azo dyes, quinone dyes,mono- and poly-oxyanthraquinone dyes, sulfur-substitutedhydroxythio-anthraquinone dyes, aminohydroxy-anthraquinone dyes,anthrapyrimidinone dyes, merocyane dyes, azomethine dyes, polycycliccompounds, benzoquinones, napthoquinones, tolanes, diphenyls,p-nitroanilines, p-nitrosodialkylanilines, dialkylaminostyroles.
 16. Apolarizing material as claimed in claim 14 wherein the polymer matrix isformed of polymeric materials selected from the group consisting of:polyimide, polyethylene, cellulose acetate, polystyrene, polycarbonate,polyester, polyacrylonitrile, polyacetal, polyacrylamide, polybutadiene,polyvinylalcohol, polymethylmethacrylate, and polyvinylcinnamate.
 17. Apolarizing material as claimed in claim 13 wherein the polarizingmaterial is provided with a coating of a material selected to alter thespectral response of said material.
 18. A polarizing material as claimedin claim 17 wherein the selected material is iodine.